Although the exact cause of concussions is not known, it is assumed that the force and duration of impact is an indication in their occurrence, and standardized testing looks at the amount of force reduction, and the increase in duration when testing a helmet's performance. When engaging in activities where impact forces to the head are likely, it is commonly required that helmets be used for additional protection. However, depending on the type of impact, helmets may be insufficient to diffuse the force and prevent damage, such as concussions. This is especially true where a user is unable to react to a force, such as when falling directly backwards, as the back of the head is impossible to protect during a fall with the arms.